X-rays from Comet C/1999 S4
(LINEAR). On July 14, 2000 the Chandra X-ray
Observatory imaged the comet repeatedly for a total of
2 ½ hours and detected X-rays from oxygen and
nitrogen ions. The details of the X-ray emission, as
recorded on Chandra's Advanced CCD Imaging
Spectrometer, show that the X-rays are produced by
collisions of ions racing away from the Sun (solar wind)
with gas in the comet. In the collision the solar ion
captures an electron from a cometary atom into a high
energy state. The solar ion then kicks out an X-ray as
the electron drops to a lower energy state.

The Chandra observations showed variations in the
amount of X-rays produced by the comet. These were
probably due to a solar flare that occurred on July 12
which would have increased the intensity of the solar
wind.

Previously, several models had been proposed to
explain how comets could produce X-rays. The Chandra
observation of C/1999 S4 proves the model described
above. Now it is possible to study the chemistry of the
solar wind, and the structure of the comet's
atmosphere.

The colors represent the intensity of the X-rays with
white being the most intense, then yellow, orange, and
red.

Note: Chandra's image of Comet LINEAR was updated to incorporate new data that brings out the faint extended emission around the comet, correctly label the position of the nucleus, and remove certain processing artifacts. The original image, released in July 2000, can be found in the More Images section. [24 Jan 03]